The GOG.com team have announced a new feature, with one-click mods available for various popular games. It's nice to see GOG focusing more on features like this and their renewed focus on supporting classic games. Giving you more of an actual reason to use them.
In addition to a few already lived, they've also announced that Skyblivion for The Elder Scrolls V: Skyrim Anniversary Edition is coming soon to GOG.

Direct Link
Mods that are already live include:
Horn of the Abyss for Heroes of Might and Magic 3: Complete. Horn of the Abyss is an expansion for Heroes of Might and Magic III that adds new factions, campaigns, creatures, artifacts, and numerous quality-of-life improvements, such as a working multiplayer lobby system.
Phobos mod for DOOM 3. The Phobos Mod for DOOM is a prequel to the original game, delivering a narrative-driven, classic-style FPS experience with modern enhancements, set during the UAC's initial experiments on Mars' moon Phobos, adding many new gameplay hours.
Vampire: The Masquerade – Bloodlines Unofficial Patch for Vampire: The Masquerade - Bloodlines. The Vampire: The Masquerade – Bloodlines Unofficial Patch does not only fixes numerous bugs left unresolved by the original developers but also restores and enhances cut content such as quests, levels, characters, and dialogue.
Fallout: London One-Click experience for Fallout 4. You already had a chance to experience how great Fallout: London is – an alternate standalone storyline set in the United Kingdom, during the apocalypse events from Fallout 4. Now though, you can boot it with just one click – no launchers, no extra steps, just pure fun.
Check out more on the GOG store.
Will be interesting to see what other major mods they put up.
In terms of doom 3 I'd rather stick to the dhewm3 build with native linux
Just downloaded phobos mod from moddb to try, sadly couldn't seem to get it working with dhewm3 :(